Terahertz control of many-body dynamics in quantum materials

Quantum-mechanical phenomena underpin the behaviour of quantum materials at the microscopic level. The description of several essential properties of these materials surpasses the generic treatment of electrons as classical non-interacting entities. Owing to the many-body nature of quantum materials, a microscopic understanding of the interactions dictating their ground state is indispensable for acquiring control over their dynamics. Non-equilibrium measurements can characterize such interactions both in time and in space, and the temporal evolution of the relaxation processes after excitation sheds light on the underlying correlations among charge, spin, orbital and lattice degrees of freedom. The energy scales of these interactions fall within the terahertz (THz) range, making THz radiation not only an effective probe but also an ideal tool for non-equilibrium perturbation, and perhaps a future tool for manipulation. In this Review, we survey how THz light has been used to drive quantum materials out of equilibrium and to retrieve information on the associated correlation processes and many-body dynamics. In particular, we show how THz light can induce superconducting-like features in layered superconductors and drive quasiparticles in heavy-fermion systems out of equilibrium. We also provide several examples of phase transitions driven dynamically by pumping correlated systems using THz light. Owing to the many-body nature of quantum materials, a microscopic understanding of the interactions dictating their ground state is essential to control their dynamics. This Review summarizes how THz light is effective for both probing quantum materials and driving them into new types of out-of-equilibrium phases.